Flash discharge tubes have conventionally been known, each of which is configured with a light-transmitting envelope and discharge electrodes which are hermetically sealed at the both ends of the envelope, with a xenon gas being sealed in the inside of the envelope. A high-frequency signal, so-called trigger signal, output from a trigger circuit is applied to the flash discharge tube via the envelope. This causes the flash discharge tube to emit a large amount of light instantaneously.
Moreover, light-emitting devices have been known, each of which is equipped with the flash discharge tube having the configuration described above, with the tube serving as a light source load. Such light-emitting devices include: stroboscopic devices used as artificial light sources for illuminating subjects to be photographed, and light-emitting devices used as eye-catching means for advertisements, for example.
As one of the firing operation modes of the light-emitting devices described above, a short-interval continuous-firing operation mode has been known in which the light-emitting device is fired many times continuously at short time intervals.
The short-interval continuous-firing operation mode is a firing operation mode in which firing is repeated at short time intervals of not longer than one second, for example. Specifically, with the stroboscopic devices described above, such a firing operation mode is used to check to see the effects (e.g. influence of shadow-castings) of illumination for photographing, in advance of the photographing. With the light-emitting devices for eye-catching of advertisements described above, the firing operation modes include a blink-firing operation mode which is used to more enhance the effects of the eye-catching of advertisements. In recent years, it has been desired for the short-interval continuous-firing operation mode to be able to offer the larger number of firing times in the continuous-firing operation.
Unfortunately, the flash discharge tube generates heat associated with the firing operation. Accordingly, during the continuous-firing operation, the heat is accumulated with an increasing number of firing times in the continuous-firing, resulting in high temperatures of the flash discharge tube. This sometimes hinders the tube from being fired, resulting in so-called misfiring.
For the reason described above, it is difficult to freely increase the number of firing times in the continuous firing. Thus, in general, a firing-halt period is forcibly introduced as needed for the flash discharge tube. Such a firing-halt period begins after predetermined firing times in the continuous-firing operation of the tube. With this configuration, the aforementioned light-emitting devices and the like can control their firing operation in the continuous-firing operation mode, thereby preventing their flash discharge tubes from being excessively heated to high temperatures.
Hereinafter, factors in hindering the firing operation of the flash discharge tube in a high temperature state will be considered.
First, an increase in charged pressure of the xenon gas is expected to be a factor; such an increase in charged pressure is due to the thermal expansion of the xenon gas that has been sealed in the inside of the envelope of the flash discharge tube. The increase in the charged pressure of the xenon gas results in restricted mobility of electrons in the inside of the envelope. In this case, generation of an arc discharge, being a firing operation, requires energies including: a larger amount of energy that is supplied to an interelectrode between the anode and cathode, and a larger amount of energy of a trigger signal that is applied to a trigger electrode. That is, such changes in the charged pressure in the high temperature state are considered to tend to hinder the firing operation of the flash discharge tube.
Moreover, a damping phenomenon of output of the trigger signal is expected to be a factor; such a phenomenon is associated with the temperature rise of the flash discharge tube.
Here, the damping phenomenon of the output will be briefly described below.
Usually, during an early period of the firing operation, an output waveform of a trigger signal in the continuous-firing operation is observed to be a vibrational waveform that has desirably-high peaks. However, as the number of firing times in the continuous firing increases, the flash discharge tube becomes in a high temperature state and tends to cause misfires, for example. In this state, the output waveform is observed to have largely-decreased peaks. This is the so-called damping phenomenon.
That is, such a damping phenomenon of the output waveform of the trigger signal means a reduction in the ability of the trigger signal to attract and induce electron emission from the cathode and to excite the xenon gas that has been sealed in. This, in turn, means a reduction in the so-called triggering ability to help the flash discharge tube start the firing operation. For this reason, in the case where the continuous-firing operation of the flash discharge tube is continued, the damping phenomenon of output of the trigger signal is considered to be a factor in causing misfires. Such a factor acts in cooperation with other factors in causing misfires, with the factors resulting from the aforementioned increase in the charged pressure of the xenon gas, for example.
Then, the present inventors consider that a factor responsible for the aforementioned damping of waveform of the trigger signal is attributed to a composition of glass that configures the envelope of the flash discharge tube, which is described below.
That is, in order for the flash discharge tube to cause an arc discharge phenomenon to emit light, electrode pins of the flash discharge tube are each commonly made with tungsten, i.e. a refractory metal material, which can withstand a large electric current that flows instantaneously through the pins. Accordingly, the envelope of the flash discharge tube is commonly made of a well-known borosilicate glass, i.e. a hard glass for use in sealing tungsten (a tungsten-sealing glass), which has a thermal expansion coefficient close to that of tungsten.
Specifically, for tungsten having a thermal expansion coefficient ranging from 4.4×10−6 K−1 to 4.5×10−6 K−1, borosilicate glass is commonly used which has a thermal expansion coefficient ranging from 3.2×10−6 K−1 to 4.1×10−6 K−1 that is comparable to that of tungsten. This configuration prevents occurrence of faults in hermetic sealing due to a difference between the thermal expansion coefficients.
Unfortunately, the borosilicate glass is known to vary in electrical characteristics as it becomes in a high temperature state. Specifically, the borosilicate glass has unfavorable characteristics in which its relative dielectric constant and dielectric loss factor increase with rising temperature, resulting in its reduced line resistance, for example. Such variations in the electrical characteristics will cause energy consumption of the trigger signal when the trigger signal is applied to the trigger electrode, in accordance with states and circumstances of the variations in the electrical characteristics.
The reason for this will be specifically described hereinafter.
First, in the case where the glass becomes in a high temperature state, the volume resistivity of the glass tube itself decreases due to factors to be described below, resulting in an increase in the amount of energy consumption of the trigger signal at the glass tube. That is, when viewed from the xenon gas inside the glass tube and an emitter impregnated in the cathode which both need energy, some amount of the energy of the trigger signal has already been consumed at the glass tube in the high temperature state. For this reason, the xenon gas and emitter in the inside of the glass tube cannot receive a sufficient amount of the energy of the trigger signal that is inputted to the glass tube. As a result, the xenon gas is not excited sufficiently, which causes misfires. Moreover, a required lighting voltage of the flash discharge tube rises.
Thus, the present inventors presume that such variations in electrical characteristics accompanying the temperature rise are a main factor responsible for the occurrence of the damping phenomenon of output of the trigger signal.
The present inventors further presume that the occurrence of the variations in the electrical characteristics described above is caused by behavior of boron and sodium, i.e. an alkaline component, in the structure of the glass tube. The boron and sodium are contained in the borosilicate glass tube. That is, in the borosilicate glass which contains sodium, i.e. an alkaline component, the mobility of sodium ions in the structure of the glass tube becomes large with increasing temperature. On the other hand, the sodium ions also function as conduction carriers. Accordingly, the larger the mobility of the sodium ions, the larger the dielectric constant as well becomes. That is, it is presumed that variations in the relative dielectric constant and the like cause the variations in the electrical characteristics described above. In addition, alkaline components volatilize from the glass tube at high temperatures, adversely causing various adverse influences on emission of light. Note that the alkaline components described above are alkali metal components including sodium and potassium; this holds for the following descriptions.
Thus, for comparison, the present inventors checked to confirm what the damping phenomenon of output is in the case where a short-interval continuous-firing operation is performed in a flash discharge tube, the envelope of which is configured with a quartz tube made of quartz glass. This confirmation has shown that, although temperature of the quartz tube rises during the operation as in the case of the borosilicate glass tube, no damping phenomenon of output of the trigger signal occurs.
Quartz glass does not contain any alkaline component, notably sodium. That is, the quartz tube does not contain any sodium ion component which functions as a conduction carrier at high temperatures. Therefore, the quartz tube shows no great change, i.e. neither large increase nor large decrease in the dielectric constant and the like. From this, it can be presumed that, because the quartz tube does not consume the energy of the trigger signal, the damping phenomenon of output of the trigger signal does not occur. From the result of the comparison described above, the present inventors also presume that the alkaline components including sodium in the borosilicate glass are a main factor responsible for the damping phenomenon of output of the trigger signal.
Note that the flash discharge tube that employs the quartz tube (quartz glass) described above is mainly made of silicon dioxide. Quartz glass has a small thermal expansion coefficient, high heat resistance, high thermal shock resistance, and high mechanical strength. The flash discharge tube configured with the quartz tube is provided with electrode pins, i.e. the discharge electrodes, which are commonly made with tungsten, a refractory metal, as in the case of that configured with the borosilicate glass tube described above.
Unfortunately, the thermal expansion coefficient of the quartz tube is approximately 0.55×10−6 K−1, while the thermal expansion coefficient of tungsten ranges from 4.4×10−6 K−1 to 4.5×10−6 K−1. That is, the thermal expansion coefficient of the quartz tube is greatly different from that of tungsten. For this reason, when the quartz tube is fixed to tungsten by welding the tube directly to the tungsten to make hermetic sealing, such a difference between their thermal expansion coefficients causes a large strain in the quartz tube, resulting in occurrence of cracks and the like in the tube.
Thus, conventionally, various methods of preventing the occurrence of cracks and the like described above have been proposed or actually used; they will be described below.
For example, a flash discharge tube has been proposed which is provided with intermediate glass bodies, the thermal expansion coefficient of which sequentially varied in a direction of the tube axis of a quartz tube (see Patent Literature 1, for example). Each of the intermediate glass bodies is configured with a plurality of glass tubes, with the different glass tubes having different thermal expansion coefficients and being disposed in order of sequential degrees of the thermal expansion coefficients in the direction of the tube axis. This configuration makes it possible to join the quartz tube to tungsten, even though they have different thermal expansion coefficients.
Specifically, for the flash discharge tube according to Patent Literature 1, the intermediate glass bodies are prepared in advance, each of which has characteristics of a plurality of different thermal expansion coefficients. Then, one end-part glass tube of each of the intermediate glass bodies is first fixed to tungsten by welding the tube to the tungsten, with the one end-part glass tube being made of a borosilicate glass which has a thermal expansion coefficient approximating to that of the tungsten.
Next, the tungsten and the one end-part glass tube of the intermediate glass body are heated together to seal them hermetically.
In the same way, the quartz tube and the other end-part glass tube of the intermediate glass body are fixed to each other by welding, with the other end-part glass tube being made of silicon dioxide, as a chief component, which has a thermal expansion coefficient approximating to that of the quartz tube.
Next, the quartz tube and the other end-part glass tube of the intermediate glass body are heated together to seal them hermetically.
With this process, the quartz tube and the tungsten are hermetically sealed indirectly via the intermediate glass body, thereby configuring the flash discharge tube in which cracks and the like are difficult to occur.
Moreover, a flash discharge tube has been known which uses aluminosilicate glass, a glass for conventional use in molybdenum sealing, which is devoid of any alkaline component such as sodium (see Patent Literature 2, for example). As in the case of the quartz glass tube, aluminosilicate glass does not contain any alkaline component and the like which functions as a conduction carrier as temperature rises. For this reason, the glass is presumed not to have large variations in its dielectric constant and like.
Patent Literature 2 discloses a fluorescent lamp which includes: a tubular bulb made of a borosilicate glass having a thermal expansion coefficient ranging from about 5.0×10−6 K−1 to about 5.5×10−6 K−1; and electrode pins made of kovar having a thermal expansion coefficient ranging from about 4.0×10−6 K−1 to about 5.5×10−6 K−1. It is noted, however, that Patent Literature 2 offers no suggestion on the aforementioned damping phenomenon associated with a temperature rise of the borosilicate glass, and that constituent materials and the like described in the Literature are nothing more than a mere example. The Literature discloses exemplified constituent elements including: the electrode pins made of either tungsten or molybdenum, for example; and an envelope made of aluminosilicate glass, for example. In the context of the examples, the Literature suggests the need for taking linear expansion coefficients (thermal expansion coefficients) into consideration. In addition, the Literature also discloses a lamp using the flash discharge tube, and a lighting device using the lamp.
That is, as described above, the light-emitting device provided with a commonly-used flash discharge tube is subjected to a damping phenomenon of output of its trigger signal. Such a damping phenomenon is caused by a high temperature state resulting from a temperature rise due to accumulation of heat generated by the flash discharge tube during a firing operation, in particular a short-interval continuous-firing operation. Such a phenomenon is responsible for occurrence of problems, such as so-called misfires in which the flash discharge tube fails to fire.
For this reason, the number of firing times in the continuous-firing operation is restricted so as not to increase. Specifically, a firing-halt period after a predetermined number of firing times in the continuous-firing operation, for example, is forcibly introduced as needed, thereby controlling the firing operation.
Hence, the flash discharge tube disclosed in Patent Literature 1 includes the envelope configured with the quartz tube made of quartz glass which is devoid of any alkaline component. This configuration can prevent the occurrence of the damping phenomenon of output of the trigger signal, allowing a great increase in the number of firing times in the continuous-firing operation. Unfortunately, the flash discharge tube described above requires the configuration in which the quartz tube is provided with the intermediate glass bodies at both end parts of the tube, in view of the difference in thermal expansion coefficients between the electrode pins and the quartz tube that serves as the envelope. This in turn requires complicated processing steps for manufacturing the intermediate glass bodies, resulting in a great increase in costs of the flash discharge tubes and the light-emitting devices.
Moreover, Patent Literature 2 offers the suggestion that the envelope employ well-known aluminosilicate glass which is devoid of any alkaline component and known for use in molybdenum sealing. However, the offered suggestion is nothing more than a mere example, as described above. That is, Patent Literature 2 discloses the technology with which molybdenum electrodes are merely used as electrodes when aluminosilicate glass is used.
Then, as in the case of the quartz tube described in Patent Literature 1, the use of aluminosilicate glass prevents the occurrence of the damping phenomenon of output of the trigger signal, allowing a great increase in the number of firing times in the continuous-firing operation. Unfortunately, the melting point of molybdenum that configures the electrode pins is approximately 2600° C. Therefore, in cases of molybdenum being used as the electrodes of the flash discharge tube that utilizes an arc discharge phenomenon in which a large electric current is generated instantaneously, the use of molybdenum is disadvantageous in view of durability and the like in comparison with the use of tungsten having a melting point of approximately 3400° C.